The present invention relates to a method of reducing magnetic and/or oxidic contaminants in lithium metal oxygen compounds in particle form in order to obtain purified lithium metal oxygen compounds.
The basic principle of rechargeable lithium-ion batteries (rechargeable accumulators) is a charging and discharging process of electrochemically active ions, whereby a source voltage is generated and the charge equalization is achieved by the migration of lithium ions. Lithium ions migrate from the cathode to the anode during the charging process. This process is reversed during the discharging process and the lithium ions migrate back to the cathode.
Graphite has often been used as anode material in rechargeable lithium-ion batteries. However, this led to the formation of a passivating intermediate layer (SEI=solid electrolyte interface) at the electrolyte boundary surface, this SEI is thermally unstable. Because of this passivating intermediate layer the internal resistance of the lithium-ion battery also increases, whereby extended charging times occur, associated with a reduced power density. In order to avoid these disadvantages attempts were therefore made to provide other anode materials.
Liquid, mostly combustible, electrolyte solutions are customarily used in lithium-ion batteries. These liquid electrolyte solutions represent a safety risk because of their combustibility and lead to increased volume of the lithium-ion batteries. In order to avoid these disadvantages attempts were made to replace these electrolyte solutions with solids by which the safety risk is minimized, and the volume of the lithium-ion batteries is reduced. Further development led to the use of solid lithium compounds as electrolytes which result in a volume reduction of the lithium-ion battery and also guarantee high intrinsic safety. A further advantage is that the solid lithium compounds can no longer dry out, whereby the longevity of the lithium-ion batteries increases.
Ceramic separators are also used as solid electrolytes, such as for example Separion® (DE 196 53 484 A1) now commercially available from Evonik Degussa, which contains ceramic fillers such as small-particle Al2O3 or SiO2.
Aono et al. investigated the lithium-ion conductivity of various materials. It was shown that doped and non-doped lithium titanium phosphates can be used as solid electrolytes because of their very good lithium-ion conductivity (J. Electrochem. Soc., Vol. 137, No. 4, 1990, pp. 1023-1027, J. Electrochem. Soc., Vol. 136, No. 2, 1989, pp. 590-591).
Systems doped with aluminium, scandium, yttrium and lanthanum in particular were investigated. It was found that doping with aluminium delivers the best results. The highest lithium-ion conductivity was demonstrated as a function of the degree of doping, as aluminium can well occupy the sites of the titanium in the crystal because its cation radius is smaller than that of Ti4+. Lithium aluminium titanium phosphates also display a low electric conductivity, which, together with their great hardness (Mohs hardness 8) distinguishes them as very suitable solid electrolytes in secondary lithium-ion batteries.
Lithium metal oxygen compounds are used not only as electrolytes, but also both as anode and as cathode in lithium-ion batteries. As lithium-ion batteries are often used in different ways in electric power tools, computers, mobile telephones etc., and these demand ever more power, the primary objective is to increase the capacity of lithium-ion batteries.
Lithium iron phosphate, used as cathode material, in combination with lithium titanates as anode, leads to a higher current-carrying capacity compared with the use of graphite with lithium titanate as anode material, above all during the charging process, and thus to an increase in the capacity of the lithium-ion battery. In addition to these advantages, these lithium-ion batteries also display high thermal and structural stability, and have a longer life. A further advantage lies in their low toxicity and the associated good environmental compatibility.
Lithium titanate is preferably used instead of graphite as anode material today (U.S. Pat. No. 5,545,468A), alternatively nanocrystalline, amorphous silicon or tin dioxide, lithium-metal compounds, magnesium molybdates or magnesium vanadates. Further anode materials are to be found in Bruce, P. G.; Scrosati, B.; Tarascon, J.-M. Angew. Chem. Int. Ed. 2008, 47, 2930-2946.
Lithium titanates crystallize cubically in the spinel structure in the space group Fd3m. Because of the structure and the potential of ca. 1.5 V versus Li/Li+, the formation of a passivating intermediate layer (SEI) on the surface of the lithium titanate spinel electrode is prevented, whereby the aging of the electrode is delayed and the number of charging processes is increased. The improved mechanical and thermal stability also leads to higher intrinsic safety of the lithium-ion batteries, whereby the tendency to short-circuit or overheat is greatly reduced.
Lithium titanates are usually produced by means of solid-state reaction over 3 h to 24 h, starting from titanium dioxide and lithium carbonate or lithium hydroxide, at from 700° C. to 1000° C. in air (U.S. Pat. No. 5,545,468A). Depending on the synthesis temperature, titanium dioxide can however still also be contained in the product in various modifications (rutile, anatase). In addition to solid-state reaction, wet-chemical synthesis of lithium titanates is also possible.
In addition to their use as anode material, lithium metal oxygen compounds are also used as cathode. Papers by Goodenough et al. (U.S. Pat. No. 5,910,382) showed that doped and non-doped lithium transition metal phosphates are particularly suitable for use as cathode material.
Lithium transition metal oxides are also used, such as for example lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, as well as doped lithium transition metal oxides and lithium transition metal phosphates such as lithium manganese nickel oxide, lithium nickel cobalt oxide or lithium iron phosphate.
Lithium transition metal oxides which have a layered structure are particularly suitable for use as cathode material, as they display a good migration capacity of lithium ions. The transition metal atom preferably has a high affinity to octahedric lattice sites, whereby the tendency towards Jahn-Teller distortion and symmetry reduction falls.
Lithium metal oxygen compounds are prepared by means of solid-state synthesis, sol-gel methods, or hydrothermal synthesis, which delivers the best results. Starting from aqueous lithium hydroxide solutions and metal salts, e.g. in the presence of a base, pure lithium metal oxides, which then still have to be annealed at high temperatures, can be obtained by precipitation of a gel-like deposit.
A precondition for the preparation of the lithium metal oxygen compounds for use in lithium-ion batteries is that their degree of purity is very high. Wet-chemical synthesis routes are preferably chosen for this, since in this way the degree of contamination by non-converted educts can be kept low. However, because of the long drying, annealing and calcining times, large agglomerated particles are obtained (particle sizes from 100 μm to 200 μm) which must be reduced by grinding processes, as only small-particle material in lithium-ion batteries leads to good specific capacity of the lithium-ion battery.
Lithium metal oxygen compounds are mostly characterized by a high hardness, there is therefore marked abrasion of the equipment and devices during grinding processes to reduce the agglomerated particles and further method steps which leads to strong magnetic and/or oxidic contamination in the lithium metal oxygen compounds.
These instances of contamination result in the discharge of the lithium-ion battery, as well as in a reduction in specific capacity. They also represent a serious safety risk, as the magnetic and/or oxidic contaminants can lead to short-circuits, whereby the lithium-ion battery is destroyed, and can even explode under certain circumstances.
In addition to contaminants resulting from magnetic abrasion of equipment, residues of non-converted educts may also be contained in the product, which also have a disruptive effect on the operation of the lithium-ion battery.
The removal of contaminants from lithium metal oxygen compounds is therefore of great importance, both in order to increase the intrinsic safety of the lithium-ion battery and to increase the specific capacity.
Various processes for removing impurities are known from the state of the art.
U.S. Pat. No. 3,685,964 discloses a method in which unwanted iron contaminants from aqueous alkali phosphate solutions are precipitated out by adding sulphides, and isolated. This method cannot be used for lithium metal oxygen compounds, as an agglomeration of the particles occurs due to the annealing and the drying, and the grinding steps that are thereby necessary lead to the appearance of magnetic and/or oxidic contaminants.
U.S. Pat. No. 4,973,458 provides a device and a method with which contaminants can be removed from gases by means of agglomeration of the unwanted contaminants and isolation by ceramic filter systems using a fluidized bed. This method is not suitable for isolating magnetic and/or oxidic contaminants from solid lithium metal oxygen compounds because, although these can be vortexed, there is a danger of their thermally induced decomposition.
The isolation of contaminants in solid phase can also be carried out as a function of the particle size (particle size of contaminant>particle size of product) in a sifting process, or using a cyclone. A purified, small-pore product is obtained, while the larger particles of the contaminants are concentrated in a sifting chamber and discarded after the sifting process.
However, once the particle sizes of the contaminant correspond to the particle size of the product as a result of a grinding process, contaminants can be removed only incompletely, as a result of which a large portion of contaminants still remains in the product.
For ground, small-particle lithium metal oxygen compounds, this method is thus not suitable for achieving the necessary degree of purity, because after the grinding treatment the particle size of the contaminant corresponds to the particle size of the lithium metal oxygen compound, and these cannot be isolated by means of a sifting process according to the method described above, as the separation capacity of a sifter or cyclone is no longer adequate.
Lithium iron phosphates also often contain contaminants consisting of metallic and/or oxidic particles due to metallic abrasion of devices during processing operations, such as grinding, caused by the hardness of the material. These contaminants in the cathode material also lead to high failure rates of the lithium-ion batteries as self-discharge processes are favoured. The removal of contaminants from lithium iron phosphates is therefore very important.
EP 09 013 035.2 describes a method which, starting from uncontaminated lithium iron phosphate, leads to the extensive removal of metallic and/or oxidic particles using a fluid-bed and sifting step. By terminating the grinding process and sifting process prematurely, metallic and/or oxidic contaminants can be isolated from the lithium iron phosphate, as for the most part these stay behind in the sifter, and can be isolated and discarded together with a residue of non-converted lithium iron phosphate.
The state of the art does not contain a method of removing magnetic and/or oxidic contaminants from small-particle lithium metal oxygen compounds.